Distillation is the process of vaporizing a substance, condensing the vapor and collecting the condensate, or distillate, in another container. This technique is useful for separating a mixture when the components have different boiling points. It is the principle method of purifying a liquid.
Four kinds of distillation techniques are available: simple, vacuum, fractional and steam distillation. Vacuum distillation, or distillation at reduced pressures, is the most efficient technique for mixtures with high boiling temperatures or with thermally sensitive components. The presence of a vacuum significantly reduces the temperatures needed for separation of the mixture. Moreover, economical low grade heat sources can be used because of the lower temperatures employed.
When using a vacuum distillation apparatus, the vacuum preferably should be generated by a liquid head rather than a vacuum pump to protect the final product's purity. However, to achieve the requisite intermediate or high vacuum with a liquid head, a significant amount of ceiling height is typically required to accommodate towers. This space is often not available.
A vacuum distillation apparatus which works well in areas of limited headroom was patented by Applicant in U.S. Pat. No. 4,444,623 on Apr. 24, 1984 and is incorporated by reference herein. However, in that apparatus the presence of air bubbles in the distillate required that the distillate pump be a positive displacement pump, such as a gear pump, which is able to pump a mixture of liquid and air. Inexpensive and high volume centrifugal pumps could not be used because the air, or other gas, tended to accumulate at the center of the impeller and block the flow of liquid through the pump, resulting in failure of the system.
Another shortcoming of the apparatus of the '623 patent is that it must be designed for operation at both start-up conditions as well as normal operating conditions. As a result, optimum operating performance is compromised for normal operating conditions.
Another vacuum distillation apparatus was disclosed by Applicant in U.S. Pat. No. 4,762,593. While the distillation apparatus in that patent can use centrifugal pumps, it does not completely overcome problems of limited available space. Rather, the total accumulated height of the bubble tubes in that apparatus must exceed 34 feet to achieve the requisite liquid head. If only three sections of bubble tubes are present, each section must reach approximately 12 feet in height. In addition, an additional foot at both the top and bottom of the distillation apparatus is typically necessary to accommodate the various pumps and condensing chambers. In an area of more limited ceiling height, shorter bubble tube sections may be used. With shorter bubble tubes, additional condensation sections are necessary to achieve the desired liquid head. As a result, the headroom problem may be resolved but the apparatus occupies additional floor space.
Therefore, a need exists for a distillation system that can be used in areas of limited space. A need also exists for a distillation system that can be designed for optimum performance in its normal running mode without a compromise in performance due to start-up conditions.